1. Field
The present disclosure relates to methods and apparatus for resolving frequency errors during transition between communication networks, and more particularly to resolving frequency errors in a mobile terminal when transitioning from one communication network to another communication network.
2. Background
In communication systems, mobile terminals sometimes have to transition or “handover” between networks employing different radio access technologies based on coverage and network settings. For instance, if a device operating within a third generation (3G) network, such as a Universal Mobile Telecommunications System (UMTS) network, lacks or is losing coverage, the mobile terminal may have need to transition from the 3G network to another radio access technology, such as a network other than a 3G network (e.g., a Global System for Mobile communication (GSM) network). When transitioning, a mobile device typically may obtain information concerning at least a couple of characteristics of the network to which the mobile is transitioning. In particular, the two determined characteristics are the power level and the timing of the network to which the mobile terminal is transitioning.
In some communication systems, determining the timing is a two-step process. In the case of a transition from a wideband code division multiple access (W-CDMA, which is used in UMTS) to a GSM network, a mobile terminal first decodes a channel having known information, such as a Frequency Correction Channel (FCCH). The Frequency Correction Channel (FCCH) is used in GSM systems, in particular, to transmit a frequency correction data burst of all “zeros” (i.e., the known information). The resulting frequency shift seen by the mobile terminal may then used for frequency correction.
After decoding the first channel, a second channel, such as a synchronization channel (SCH), is decoded based on the decoded information from the first channel for purposes of decoding the transmitted payload or data. In GSM, for example, the Synchronization Channel (SCH) is a downlink only channel that is broadcast periodically to help mobile terminals identify the base station information code (BSIC) and the time division multiple access (TDMA) frame number.
Frequency error, however, can arise when transitioning from one network to another due to factors such as Doppler effect, arising due to the speed of the mobile terminal, and the difference in the local oscillator clock between the different network base stations (e.g., Frequency error=Doppler frequency+clock synchronization between UMTS and GSM systems). Performance of SCH decoding, in particular, is affected by the frequency error. Typically, the performance degrades for frequency errors greater than 300 Hz, resulting in SCH decode failures. Without decoding the SCH, the mobile terminal does not have the BSIC information to uniquely identify the GSM cell. Considering that the inter-network handover is triggered only when the coverage of the first network is weak, this will result in out of service or poor call performance.
Accordingly, in order to improve the SCH decoding performance, known mobile terminals employ frequency correction techniques. These techniques may involve the use of digitally controlled crystal oscillators (DCXOs). DCXOs compensate for frequency errors using a combination of digital and analog circuitry. Frequency errors may be programmatically controlled using a control loop topology, as an example. This methodology, with further implementation by DCXOs, however, is complex and is power inefficient because the FCCH channel is decoded each time prior to decoding the SCH channel.